In computer apparatus and televisions as well, cathode-ray tubes (CRTs) are often used to provide visual signal image output. The generalized modern CRT consists of an electron-beam-forming system, electron-beam deflecting system, phosphor screen, and evacuated envelope. The electron beam is formed in the electron gun, where it is modulated and focused, and then travels through the deflection region, where it is directed toward a specific spot or sequence of spots on the phosphor screen. At the phosphor screen the electron beam gives up some of the energy of the electrons in producing light or other radiation, some in generating secondary electrons, and the remainder producing heat. Typically, the screen has a convex surface as opposed to a flat surface.
In producing a picture upon the CRT screen, a succession of horizontal scan lines (providing a sequence of dots) is generated. Typically, the horizontal scan lines move from the upper left corner of the picture and proceed to the right with a slightly downward orientation. This succession of active scan lines and active retraces continues until a point near the middle of the bottom of the picture is reached. The spot then rapidly moves upward to the midpoint of the top of the frame, and the downward scan and retrace motions are repeated. A problem occurs in that signals for generating the horizontal scan lines are provide on the basis that the screen is flat, when in fact it is convex to varying degrees depending upon the CRT. Such a difference causes distortion in the horizontal focus which is observed by a viewer when the horizontal scan line signal is not accordingly compensated for delivery to a convex screen.
In conventional technology, the horizontal focus controller compensates for the distort on of the CRT screen by generating a parabola signal corresponding to a curved surface the CRT screen and outputting it with a horizontal scan signal. Additionally, a horizontal parabola signal should be synchronized with a flyback pulse to control such a horizontal focus. The flyback pulse is generated when the electron beam reaches the right edge of the CRT. The flyback pulse causes the beam to be rapidly deflected from the right edge of the CRT to the left edge so that the beam may move across the screen from left to right again.
An input synchronizing signal includes both the horizontal and vertical synchronizing signals In the vertical direction, a vertical synchronizing signal occurs once for each horizontal scan. Since a horizontal portion of the CRT has a higher operating frequency than a vertical portion of the CRT, the output gain and the phase of the parabola signal should hardly change based upon the vertical synchronizing signal. Also, since the CRT operates in a multi-synchronizing method, the above-mentioned conditions should be met with respect to all the horizontal synchronizing signal frequencies. While there is no problem based upon the vertical synchronizing signal, there is however, problem in that the output gain and the phase of the parabola signal change according to the initial input synchronizing signal frequency in a conventional technology. Accordingly, an unstable parabola signal will no properly compensate for the distortion due to the convex surface of the CRT and cause a distorted screen on the CRT.